Patent GB-1438071-A (GB)
Title: Method for obtaining video signals and apparatus for performing the method
IPC: H04N9/07
Filed: 1974-01-03
Abstract: 1438071 Colour television K VOCKENHUBER and R HAUSER 3 Jan 1974 [3 Jan 1973] 00205/74 Heading H4F The output of a pick-up tube having associated therewith a strip filter having a repeating sequence of transparent (Y), cyan (Y-R), transparent (Y), and yellow (Y - B) strips is processed by dot-sequential methods to provide luminance (Y), red (R) and blue (B) signals which are fed to a matrix 62 from which are derived red (R), green (G), and blue (B) signals. A differentiating circuit 35 and threshold switch 36 detect sudden changes in the tube output which are indicative of colour edges in the viewed scene, and control the operation of the processing circuitry to eliminate errors in the colour signals which would otherwise occur; differentiating circuit 35 is connected to receive the input to a delay 33 providing a delay of one image dot so as to provide time for circuits 35, 36 to correctly control the processing circuitry. Switches 41, 42, 43, 44 are successively closed, switch 41 passing a luminance signal (Y 1 ) at time t 1  to a one-dot delay 45 from which it is fed, at time t 2 , to an adder 46 which also receives, via switch 42, a difference signal Y - R ; adder 46 thus produces a red signal R1 which is passed via a one-dot delay 47, a switch 48 and a two-dot delay 59 to matrix 62 via a low-pass filter 61. A blue signal is similarly derived at time t 5  via one-dot delays 52, 54, adder 53 and switch 55. If a colour edge which crosses a transparent strip is detected by circuits 35, 36 the following colour signal produced by adder 46 or 53 would be incorrect, as the luminance signal Y derived from this transparent strip would not equal the luminance component of the difference signal Y - R or Y - B. Circuits 35, 36 therefore cause, on detection of a colour edge, changeover of switches 48, 55 from their positions shown so that, e.g. switch 48 passes, at time t 3 , a signal R 2  derived by adder 51 from the difference signal Y-R delayed by one-dot delay 50 and the following luminance signal Y 2  passed by switch 43 at time t 3 ; a blue signal B 2  is similarly derived by adder 58 from signals Y - B and Y 5  and passed to switch 55 at time t 5 . To provide a luminance signal at all times a switch 66 connects a four-dot delay 67 alternately to a one-dot delay 63 and an adder 64, the adder 64 receiving the output of delay 63 and the output of a switch 65 alternately passing a red signal from switch 48 at time t 3  and a blue signal from switch 55 at time t 5 . Adder 64 therefore derives at time t 3  the luminance signal R + (Y - R) and at time t 5  the luminance signal B + (Y - B). Switches 41-44 may be controlled by the output of a limiting amplifier (21, Fig. 2A, not shown) passed via a distributer, this amplifier receiving the tube output and deriving pulses therefrom; black index strips are provided in the filter, at least at the beginning of each line scan. Alternatively the horizontal scanning frequency of the tube may be controlled by a phase comparator (26, Fig. 3, not shown) receiving outputs from a reference oscillator (27) and a limiting amplifier (25) receiving the tube output, black index strips also being provided in the filter.